How should he pay zakaah on his savings from his monthly salary?

Question

I have been working for a company for approximately two years, and I save a large part of my salary every month. In the tenth month, I had accumulated the equivalent of the nisaab (minimum threshold at which zakaah becomes due) and the amount keeps increasing and decreasing, but it has not fallen below the minimum threshold. Now it is the twenty-first month and the amount is almost double and the nisaab (minimum threshold) has changed. I am confused and no longer know what I should do. What is the nisaab (minimum threshold) I should take into account, the old one or the new one? What is the amount (of savings) I should take into account? Is it the latest one or the one for which a (hijri) year has passed? Please clarify.

The easiest and safest way to work out the zakaah on what a
Muslim has saved from his monthly salary is as follows:

Determine the month in which what you had saved from your
salary reached the minimum threshold (nisaab) at which zakaah becomes due.
Then when one hijri year has passed since this threshold was reached, pay
zakaah on all the money you have, even what you have saved from your last
month’s salary which has only been in your savings for a few days.

Thus zakaah will have been paid for the amount for which a
year has passed.

As for the savings from the months that came after that, you
will have paid their zakaah in advance, and paying zakaah in advance is
permissible; there is nothing wrong with it.

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
With regard to zakaah on monthly salaries, the best, easiest and safest way
of paying it is to choose a particular month to count your wealth and pay
zakaah on all of it.

For example: a man has the habit, every time the month of
Ramadan begins, of counting what he has and paying zakaah on it, and he even
pays zakaah on the salary for the month of Sha‘baan which comes before
Ramadan. This is good and in fact gives peace of mind; we have not found
anything easier than this.

If someone were to say that only a few days have passed since
acquiring the wealth in Sha‘baan, we would say: it is zakaah that is paid in
advance, and it is permissible for a person to pay zakaah one or two years
in advance.

In that case we say that the best thing is for a person to
choose a particular month in which he counts all his wealth and pays zakaah
on that for which one year has passed and that for which one year has not
yet passed.

End quote from Majmoo‘ Fataawa wa Rasaa’il al-‘Uthaymeen,
18/175

We have previously published a number of fatwas which explain
how to pay zakaah on salaries. See questions no.
26113,
50801,
94314